ruby-on-rails - 这是对多态性的有效使用吗？如果是，我应该如何声明这种关系？

Question

我有一个 Appointment 模型，可以由 Tutor 或 Student 初始化。一旦被一方初始化，另一方可以接受或拒绝。

我将我的模型设计为:约会和参与者。 Participant 有两个属性:participant_id 和 participant_type ("Tutor"/"Student")。我想声明 Appointment has_one Tutor, has_many Students 使用多态。

我的问题是:这是对多态性的有效使用吗？如果是，那么我应该如何声明这种关系和外键？如果不是，那是为什么？

谢谢。

Answer 1

当您拥有在不同实体(例如学生和导师)之间共享的共同属性(例如参与约会的能力)时，请使用多态性。我认为你的情况需要参与者的多态性，而不是约会。

问问自己:是否有不同类型的约会或不同类型的参与者？从您提供的信息来看，您似乎有一种约会，以及不同种类的参与者。

多态参与者的例子

预约

class Appointment < ActiveRecord::Base
  has_many :participants
  has_one  :tutor,    :through => participants
  has_many :students, :through => participants
end

学生

class Student < ActiveRecord::Base
  has_many :appointments, :as => appointable
end

导师

class Tutor < ActiveRecord::Base
  has_many :appointments, :as => :appointable
end

参与者

# This model joins your appointable entities (Tutors and Students)
# to Appointments
class Participant < ActiveRecord::Base
  belongs_to :appointment
  belongs_to :appointable, :polymorphic => true
end

就声明外键而言，Rails 会为您处理。

参与者迁移

class CreateParticipants < ActiveRecord::Migration
  def up
    create_table :partipants do |t| 
      t.references :appointment
      t.references :appointable, :polymorphic => true
    end
  end

  def down
    drop_table :participants
  end
end

为了更好地理解 Rails 如何将诸如 polymorphic 之类的关键字转换为 SQL 关联，请参阅指南:http://guides.rubyonrails.org/association_basics.html#polymorphic-associations

我认为状态机是一个有趣的选择——我没有任何 Ruby/Rails 状态机项目的经验，所以我不能就此给你建议。

调度

这是关于如何设置日程安排的不完整描述。希望这足以让您入门。

将这些方法添加到约会:

class Appointment < ActiveRecord::Base

  # Anybody may request a new appointment,
  # specifying the initiator, and other participants
  # they would like to attend.
  def self.request requester, requested_participants=nil
    a = self.new
    a.status = "requested"
    a.requester = requester
    a.request_participation_of requested_participants
    a.save!
  end

  # Anybody may call this method
  # to request that the appointment be 
  # rescheduled, specifying the requester
  def reschedule requester
    self.status = "reschedule_requested"
    requester.participation_in(self).update_attribute :status=> "requester_of_reschedule"
    self.participants.where("appointable_id != ?", requester.id)
      .update_all :status => "reschedule_requested"
    self.save!
  end

  protected

  def requester= requester
    requester.participation_in(self).update_attribute :status => "requester"
  end

  def request_participation_of participants
    if participants.is_a? Array
      participants.each do |participant|
        participant.request_participation_in self
      end
    else
      request_participation_of [participants]
    end
  end
end

Scheduling 模块包含 Tutors 和 Students 的方法，因此您可以执行 student_3.request_appointment tutor_1 或 tutor_1.reschedule_appointment appointment_4 等操作。

lib/appointments/scheduling.rb:

module Appointments::Scheduling
  # When a Student or Tutor's participation
  # in an Appointment has been requested,
  # he/she indicates acceptance of the request
  # with this method
  def accept_participation_in appointment
    self.participation_in(appointment).update_attribute :status => "accepted"
  end

  # Same as above, to decline the request
  def decline_participation_in appointment
    self.participation_in(appointment).update_attribute :status => "declined"
  end

  # This method finds the Student or Tutor's
  # Participant object for a particular Appointment
  def participation_in appointment
    Participant.where(:appointment_id => appointment.id)
      .find_by_appointable_id self.id
  end

  # A student or tutor can request a new
  # Appointment with a participant or
  # group of participants with this method
  def request_appointment participants
    Appointment.request self, participants
  end

  # This Student or Tutor's participation
  # in an appointment can be requested with
  # this method
  def request_participation_in appointment
    Participant.find_or_create_by_appointment_id_and_appointable_id(
      :appointment_id => appointment.id,
      :appointable_id => self.id
    )
  end

  # This Student or Tutor's confirmation of
  # a scheduled Appointment may be requested
  # with this method
  def request_reschedule_of appointment
    new_status = "reschedule_requested"
    if participant.id.eql? requester.id
      new_status = "requester_of_reschedule"
    end
    self.participation_in(appointment).update_attribute :status => new_status
  end

  # A Student or Tutor may reschedule a
  # particular Appointment with this method
  def reschedule_appointment appointment
    appointment.reschedule self
  end
end

一旦这些模块就位，您就可以将它们包含在适当的实体中:

class Appointment < ActiveRecord::Base
  include Appointments::Schedulable
end

class Student < ActiveRecord::Base
  include Appointments::Scheduling
end

class Tutor < ActiveRecord::Base
  include Appointments::Scheduling
end

我的示例还要求您向 Appointment 和 Participant 添加一个 status 字段。我最终会创建一个 AppointmentStatus 和一个 ParticipantStatus - 但是，首先我会让系统在没有它的情况下工作。

这里有一个有用的资源，可以帮助您创建用于模型的模块:http://henrik.nyh.se/2008/02/rails-model-extensions